It is well known in the engine arts to improve internal combustion engine performance by variably re-circulating a portion of the engine exhaust back into the intake air stream for re-combustion with the fresh air/fuel mixture. The variable control device typically provided for this function is known in the art as an exhaust gas recirculation (EGR) valve.
The EGR process is chemically and mechanically demanding on EGR valve components. The condensates carried in the exhaust gases can be very corrosive of some types of metals in the EGR valve and in the exhaust system.
In a prior art EGR valve, because the exhaust gas pressure is higher in the valve group assembly than the actuator environment pressure (typically one atmosphere), exhaust gas flows into the space between the pintle shaft and the bushing, carrying carbon, corrosive condensate, and other contaminants into this clearance. In internal combustion engines generally, and especially in diesel engines, such contamination of the bushing/pintle shaft interface results eventually in pintle shaft sticking and degradation of components in the assembly.
Two principal causes of degradation by sulfuric acid condensate attack have been found to be a) bushing corrosion on the typical/common EGR bushing materials; and b) corrosion of iron components in the exhaust system. In diesel engines, these causes are abetted by the fact that the exhaust typically is significantly cooled by aftertreatment devices before reaching the EGR valve. Thus, corrosive compounds are readily precipitated onto the valve bushing components, especially when the engine is shutdown and those compounds are allowed to stand in contact with the bushing components. The pintle shaft may become stuck to the bushing, resulting in valve failure when the engine is restarted, accompanied by potential damage to the valve actuator.
This situation is especially worrisome for diesel engines employed in off-road activities, such as for powering farm tractors. The Federal standards for sulfur content in diesel fuel permit only 15 ppm for highway use, but for off-road use sulfur levels may be as high as 5000 ppm, resulting in very high levels of H2SO4 in the engine exhaust.
Further, in prior art EGR valves having a ring shaft seal, the seal typically is disposed between the valve group assembly and the actuator, the intent being to keep exhaust gases out of the actuator. Such a seal location, however, does nothing to protect the bushing and bushing/pintle shaft interface from exhaust gas attack.
What is needed in the art is an EGR valve having a bushing arrangement wherein a) materials of the wear surfaces are immune to corrosion compounds in exhaust gases; and b) significant volumes of exhaust gases are substantially prevented from reaching bushing/pintle shaft surfaces.
It is a principal object of the present invention to prevent failure of EGR valves due to bushing failure by exhaust gas corrosion in internal combustion engines.